NAME

       ArcFS - Archive file format for the RISC OS ArcFS filesystem

CONVENTIONS

       Strings are given in ASCII with C-style escapes. No terminating   is present if not explicitly included.

       Words are 32 bit little-endian numbers.

       Half words are 16 bit little-endian numbers.

       This  document  describes format version 0, for ArcFS minimum versions 0.40 and 1.00, and version 0a, for
       ArcFS minimum versions 0.58 and 2.17.

FORMAT

       The file consists of a file header, a sequence of archive headers, then a sequence of compressed files.

FILE HEADER

       File string signature
       The string "Archive" followed by a zero byte.

       Header length
       Length of header info in bytes, one word, multiple of 36.

       Data offset
       Offset, in bytes, into archive at which data starts, one word.

       Read-only version
       Minimum read-only version required, multiplied by 100, one word.

       Read-write version
       Minimum read--write version required, multiplied by 100, one word.

       Reserved
       Reserved for future use, 68 bytes.

ARCHIVE HEADER

       The last header starts at 60+length of headers and ends at 96+length  of  headers.  The  format  of  each
       header is as follows.

       Info byte
       Object information byte, show compression method used:
            0x00 end of directory marker
            0x01 object has been deleted
            0x82 stored
            0x83 packed
            0x88 crunched
            0xFF compressed

       File name
       File name, 11 bytes.

       Full length
       Original file length, or 0xFFFFFFFF for a directory), one word.

       Load address
       Load address of the file, or 0xFFFFFFdd for a directory), one word.

       Exec address
       Exec address of the file, or 0xdddddddd for a directory), one word.

       File load and execution address may contain a filetype and datestamp as normal.

       Directories can be date-stamped, the number of centiseconds since the 1st January 1900 when the directory
       was created are given by &dddddddddd contained in the directory's load and execution addresses  as  shown
       above.

       Archives  converted from spark(5) format using ArcConv will have the same full length, compressed length,
       load address and execute address as they had in the Spark archive.

       Attributes
       Attributes of the file, one word.  Contains the number of bits compression for crunch and compress format
       files and the least significant bytes contains the standard RISC OS information on file access.

       Compressed length
       Length of compressed data, or 0xFFFFFFFF for a directory, one word.

       Information word
       The  top  bit (definitively) indicates whether the object is a file or a directory, 0 for a file, 1 for a
       directory.

       In the case of a file, the least significant 31 bits show the offset from the start of data at which  the
       file data can be found.

       In  the case of a directory, the least significant 31 bits show the offset from the start of headers (96)
       at which the next object in this directory can be found, the following object being the first in the sub-
       directory.

FORMAT VERSION 0a

       This  archive  format is backwards compatible with versions 0.40 and 1.00, the only difference being that
       there is a block of catalogue information immediately prior to the start of each file's data as described
       below.

       Note  that this information is there to aid the recovery of most files in the event of archive corruption
       and is not used by ArcFS.

       Note, also that Compact and Autocompact in release versions 1.09, 2.02 and  2.11  will  compact  out  the
       catalogue information blocks, so try to avoid writing to new archives with old versions of ArcFS.

       Marker
       A marker to indicate the start of a file, the string "FileData", not terminated.

       Info byte
       One byte, as above.

       Leaf name
       The file name, 11 bytes.

       Full length
       One word, as above.

       Load address
       One word, as above.

       Exec address
       One word, as above.

       Attributes
       One word, as above.

SEE ALSO

       spark(5), nspark(1)

                                                                                                        ARCFS(5)